Republican lawmakers from agricultural states have warned Mr. Trump of potential repercussions in the 2018 midterms if he leaves the trade pact

The future of the North American Free Trade Agreement, which binds the economies of the U.S., Canada and Mexico, has rarely looked as murky as it does right now.

Negotiators missed a self-imposed deadline to rewrite the deal in May. Then a quarrel between U.S. President Donald Trump and Canadian Prime Minister Justin Trudeau after G-7 meetings in Quebec this month put key players at odds with each other just as negotiators were trying to decide what steps to take next.

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